The present invention relates to non-destructive testing and, more particularly, to ultrasonic inspection of material bodies for defects. A major objective of the present invention is to provide for economical and effective inspection of nuclear reactor vessel nozzles.
Nuclear reactors must be routinely inspected for defects that could result in leakage from a reactor vessel if left undetected and allowed to grow. One relatively economical and effective inspection method uses ultrasonic examination. Generally, ultrasonic inspection involves directing an ultrasonic beam into a volume to be inspected and monitoring reflections of the beam. The location of a boundary causing a reflection can be determined from the time between pulse transmission and the detection of the reflection. A reflection from within the material body may be an indication of a defect. Further ultrasonic testing from successive transducer positions can confirm and characterize a defect.
Nuclear reactor nozzles pose a severe challenge to ultrasound techniques. In particular, the physics of ultrasound makes it difficult to provide reliable inspections of the inner radii of reactor nozzles. For example, one type of boiling water reactor (BWR) has several feedwater return nozzles disposed about the circumference of a reactor vessel slightly above the reactor core. The inner radius of such a nozzle can be subject to considerable thermal stress, mechanical fatigue, and potential cracking. Ideally, this inner radius would be inspected from inside the reactor or inside the nozzle. However, the required reactor disassembly, the requirement of removing reactor water, and the potential exposure to radiation from the core make this undesirable.
Economical ultrasound testing requires that the interior defects be inspected from the exterior of the vessel. This means that the transducer that emits the ultrasound beam and the transducer that detects the reflections (which are usually the same transducer, but can be different transducers) must be on the vessel exterior. The emitting transducer must be positioned and oriented to direct energy to a volume to be inspected. The receiving transducer must be positioned and oriented to detect reflections from potential defects in that volume. Since defects are typically of unknown dimensions and orientation, it is preferable to expose them to diverse ultrasound beams to increase the likelihood of detection of defects and to enhance characterization of detected defects.
Standard ultrasound techniques allow effective inspection of certain geometries. For example, defects at the interior wall of a cylinder can be effectively detected from the cylinder's exterior. Nozzle bores, for example, have a cylindrical geometry, and are thus subject to reliable inspection. However, volumes at the nozzle inner radius and adjacent bore are accessible from fewer positions so that defects can be "illuminated" from fewer angles. Thus, defects more readily escape detection and are more difficult to characterize. In particular, radially-oriented defects, which experience shows to be common, are difficult to detect using conventional techniques. Accordingly, resort has been made to alternative inspection methods, including dye penetration techniques. However, these are much less convenient and economical. What is needed is an effective method for ultrasonically inspecting reactor nozzles from the vessel exterior.